In mounts for optical systems, it is desirable to observe basic principles of kinematics. A body in space, such as a lens or mirror, has six degrees of freedom or ways in which it may move: translation along the three rectangular coordinate axes, and rotation about these three axes. A body is fully constrained when each of these possible movements is singly prevented from occurring. However, it is sometimes desirable in an optical system for some degrees of freedom to be allowed, and so semikinematic methods can be used.
Meanwhile, for optical systems which must withstand a difficult thermal and/or vibration environment, a conventional form of mount is achieved by oversizing the fit between a mount structure and the optical element and cementing the optical element in place with a relatively thick bond such as a compliant, elastomeric type of cement. This technique can be useful for large elements where the thermal expansion difference between the element and the mount is a serious problem, and the relatively thick bond allows for expansion and contraction that is accommodated by the compliant cement. Such a compliant and thick bond minimizes the stress that is induced on the optical element due to thermal expansion, which can thereby protect against degraded imaging performance.
An example is shown in FIGS. 1A and 1B which are top and side views, respectively, of an optical element 102 mounted in accordance with the prior art. Optical element 102 is, for example, a mirror in a Cassegrain mirror system for a telescope and is comprised of a material such as glass. As shown, element 102 is mounted to a structural element such as hub 104 with bonding material 106.
For applications where light weight is desired, hub 104 can be comprised of a material such as aluminum. This material has a different thermal expansion coefficient than glass, of which the optical element typically is comprised. To keep the stress low through the possible range of expansion of these mount and optical element materials, therefore, the bond material is comprised of an silicone rubber material such as Silastic E RTV, trademarked and available from Dow Corning, and needs to be relatively thick.
This technique for mounting optical elements suffers from several problems. For example, an optical system employing mounts using this technique can suffer from positional sag due to gravity. More specifically, the thickness and relative softness of an elastomer bond can not support the self weight of the optical element and thus, can not maintain the position of the optical element. Moreover, such mounts can suffer from poor thermal performance because elastomers like Silastic E RTV have a very high thermal expansion coefficient compared to metals and glass. Accordingly, for a thick bond having such a high thermal expansion, the motion over a large temperature range can be very significant. Relatedly, systems having such mounts exhibit a low resonant frequency resulting in motion of the optical element when subjected to external loads like a rotating helicopter blade or turbulence on a jet plane.
Accordingly, a need remains in the art for a low stress optical mount that does not suffer from problems afflicting the conventional approaches.